The present invention relates to self-elevating platforms for oil-drilling and relates more particularly to production-drilling platforms for production operations at sea. Self-elevating platforms for carrying out drilling at sea were used until recently for depths less than 100 m.
However, to increase the field of application of this type of platform, some constructors are putting forward concepts which relate to standard architecture of self-elevating platforms in order to reach depths greater than 100 to 110 m. This results in a considerable increase in the leg and body dimensions and of the displacement for towing.
In this case, lattice type legs result, or which the distance between the members can be greater than 20 m.
The increase in the leg dimensions results in a considerable increase in weight, in a higher position of the center of gravity when towed, and therefore in a loss of stability as well as greater hauling forces when being towed.
Over and above the economic aspect, this makes difficult the moving of such a platform from one drilling site to another.
Drilling operators prefer to work with self-elevating platforms, which are "fixed" platforms when in the elevated drilling position on their legs, rather than with semi-submersible platforms which are mobile platforms.
This is why other constructors propose to move the boundary of use of the self-elevating platforms up to depths of 130/150 m and beyond, by placing an individual and independent raising block between the leg of the platform and the sea-bed in order to increase the installation depth of the platform.
However, a self-elevating platform does not work at the maximum depth that it can reach and a platform designed for a depth of 130/150 m is often used at depths which are less than this.
In this type of platform, the ends of the legs have feet for resting on the blocks which are no longer adapted for resting directly on the sea-bed.